The first amplitude modulated signal was transmitted in 1901 by a Canadian engineer named Reginald Fessenden. He used a continuous spark transmission and placed a carbon microphone in the antenna lead. This transmission was very crude, signals were audible over a distance of a few hundred metres. The quality of the audio was not good. In amplitude modulation, angular frequency ω and the phase Φ are kept constant and the amplitude A_c of the carrier wave is varied in accordance with the modulating wave. When the amplitude of a high-frequency carrier wave is changed in accordance with the intensity of the signal, it is called amplitude modulation.

Amplitude modulation is done by a circuit called modulator. In amplitude modulation, the amplitude of the carrier is varied in accordance with the information signal. Here we explain the amplitude modulation process using a sinusoidal signal as the modulating signal.

We can see that during the positive cycle of low-frequency signal and the positive cycle of carrier wave the amplitude of carrier wave increased, while in the negative cycle it is decreased.

Features of Amplitude Modulated Wave:

• The amplitude of carrier wave changes according to the intensity of the signal.
• The variation in amplitude of a carrier wave is at the frequency of the signal f_m.
• The frequency of amplitude modulated wave is the same as that of the carrier wave fc.
• The amplitude of amplitude modulated wave is not constant but it has a similar sinusoidal variation as that of the signal wave. Thus the amplitude-modulated wave is loaded with the information contained in the low-frequency signal message.
• As amplitude modulation is simple it is widely used.

Modulation Factor or Modulating Index:

The ratio of change of amplitude of the carrier wave to the amplitude of the normal carrier wave is called the modulation factor. It describes the depth of modulation i.e. the extent in the variation of the amplitude of carrier wave due to the signal.

The value of the modulation factor depends upon the amplitude of the carrier wave and the signal.

Importance of the Modulation Index:

• It is an indicator of the level of modulation.
• If there is too low a level of modulation then the amount of variation in carrier amplitude is small. Thus the audio signal being transmitted will not be very strong. Hence the modulation does not utilize the carrier efficiently.
• If there is a too high level of modulation then the carrier can become over modulated causing sidebands to extend out beyond the allowed bandwidth causing interference to other users. Hence there will be distortion during the reception.

Amplitude modulation with modulation factor 0.5 or 50%

Amplitude modulation with modulation factor 1 or 100%

When the modulation index reaches 1.0, i.e. a modulation depth of 100%, the carrier level falls to zero and rise to twice its non-modulated level.

Amplitude modulation with a modulation factor greater than 1 or greater than 100%

Any increase of the modulation index above 1.0, i.e. 100% modulation depth causes over-modulation.

The carrier experiences 180° phase reversals where the carrier level would try to go below the zero point. Due to this phase reversals, there is a rise in additional sidebands resulting from the phase reversals (phase modulation) that extend out, to infinity theoretically. This may cause serious interference to other users if not filtered.

Broadcast stations, take measures to ensure that the carries of their transmissions never become overmodulated. The transmitters incorporate limiters to prevent more than 100% modulation. They incorporate automatic audio gain controls to keep the audio levels near 100% modulation for most of the time.

Expression For Modulation Index:

The modulation index (μ_a) of an amplitude-modulated wave is defined as the ratio of the amplitude of the modulating signal (Em) to the amplitude of the carrier wave (E_c).

μ_a = (E_m)/(E_c)

To avoid distortion E_m < E_c

For modulated wave

μ_a = (E_max – E_mmin)/(E_max + E_min)

If for AM wave the maximum amplitude is ‘a’ while the minimum amplitude is ‘b’

E_max =  (E_c + E_m) = a …………. (1)

E_min =  (E_c – E_m) = b …………. (2)

Solving equation (1) and (2) we get

E_m = (a – b)/2 and E_c = (a + b)/2

μ_a = (E_m)/(E_c) = (a – b)/(a + b)

Demodulation:

The AM signal received is passed through a demodulator to extract the information being carried by it. The process of separating or extracting the modulation from a signal is called demodulation or detection. Demodulation of AM signals can be done using simple circuits consisting of diodes.

Advantages Of Amplitude Modulation:

• It is simple to implement.
• Demodulation of AM signals can be done using simple circuits consisting of diodes.
• AM transmitters are less complex.
• AM receivers are very cheap as no specialized components are needed.
• AM waves can travel a longer distance.
• AM waves have low bandwidth

Disadvantages of Amplitude Modulation:

• An amplitude modulation signal is not efficient in terms of its power usage. Power wastage takes place in DSB-FC (Double Side Band – Full Carrier ) transmission.
• It is not efficient in terms of its use of bandwidth. It requires a bandwidth equal to twice that of the highest audio frequency. In amplitude modulation sidebands contain the signal. The power in sidebands is the only useful power. For 100 % modulation, the power carried by AM waves is 33.3 %. The power carried by the AM wave decreases with the decrease in the extent of modulation.
• AM detectors are sensitive to noise hence an amplitude modulation signal is prone to high levels of noise.
• Reproduction is not high fidelity. For high fidelity (stereo) transmission bandwidth should be 40000 Hz. To avoid interference the actual bandwidth used by AM transmission is 10000 Hz.

In spite of the disadvantages amplitude modulation is still in widespread use for broadcasting on the long, medium and short wave bands, some mobile or portable communications systems including some aircraft communications.

Applications of Amplitude Modulation:

Broadcast transmissions: 

AM is still widely used for commercial broadcasting on the long, medium and short wave bands because the radio receivers capable of demodulating amplitude modulation are cheap and simple to manufacture. The atmospheric signals like lightening and man-made electrical signals affect this transmission.

Air-band radio:  

VHF transmissions for many airborne applications still use AM. It is used for ground to air and ground to ground radio communications. e.g. television standard broadcasting, aids to navigation, telemetering, radar and, facsimile. etc.

Single sideband:  

Amplitude modulation in the form of single sideband is still used for point to point HF (high frequency) radio links.

Quadrature amplitude modulation: 

AM is widely used for the transmission of data in everything from short-range wireless links such as Wi-Fi to cellular telecommunications and much more. Quadrature amplitude modulation is formed by mixing two carriers that are out of phase by 90°.

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Before the concept of the modulation, the signal used to be sent in form continuous wave with periodic interruption as in morse code. To code, send and decode such message high expertise was required. The direct current is not suitable for transmitting the data because of its steady nature. A continuous AC wave cannot be used for transferring data as all the variations or cycles are alike. Thus continuous AC wave in the form of a series of pulses can be used to carry data effectively.

A high-frequency carrier wave is used to carry the audio signal. The audio signal is superimposed over the carrier wave. This process is called modulation.

Modulation may be defined as the process of changing some characteristics like amplitude, frequency or phase of a carrier wave in accordance with the intensity of the signal is known as modulation.

The Need for Modulation in Communication System:

The purpose of a communication system is to transmit information or message signals. Message signals are also called baseband signals, which essentially designate the band of frequencies representing the original signal, as delivered by the source of information. No signal, in general, is a single frequency sinusoid, but it spreads over a range of frequencies called the signal bandwidth.

To understand the need for modulation let us suppose that we wish to transmit an electronic signal in the audio frequency (AF) range (baseband signal frequency less than 20 kHz) over a long distance directly. Let us find what factors prevent us from doing so and how we overcome these factors.

Size of the Antenna or Aerial:

For transmitting signals antenna is required whose length should be at least (λ/4), to sense the time variation of the signal.

c = υλ

∴  λ = c/υ

Length of antenna = λ/4 = c/4υ  = 3 x 10⁸ / (4 x 20 x 10³)  =  3.75 x 10³ m = 3.75 km

The antenna size for considered baseband signal frequency is of impracticable length. Hence direct transmission at baseband signal frequency is not practical. If we used a frequency say 1MHz, then

Length of antenna = λ/4 = c/4υ  = 3 x 10⁸ / (4 x 1 x 10⁶)  =  75 m

This is a practical length of the antenna.

Thus we can obtain transmission by use of practical antenna length using high frequency for transmission. Thus there is a need for translating the original low-frequency signals into high frequency before transmission.

Effective Power Radiated by Antenna:

The power radiated is proportional to (l /λ)². Where ‘l’ is the size of the antenna. Thus for the same antenna, the power radiated is inversely proportional to the wavelength of the wave i.e. directly proportional to the frequency of the wave. Thus the effective power radiated by long-wavelength is small.

For good transmission of signal high power is required. Hence direct transmission at baseband signal frequency is not practical. To get more power radiated the high-frequency transmission should be used.

Mixing Up of Signals From Different Transmitters:

For considered baseband frequency there are full chances of mixing up of the signals from the transmitters operating in the same band. It is just like many people talking simultaneously. Thus, all these signals will get mixed up and there is no simple way to distinguish between them. Hence bands are allotted to different types of broadcast and communication systems.

Operating range:

The energy of a wave is directly proportional to the frequency of the wave. For the wave to be propagated over a large distance, the wave should possess more energy. It is only possible using a higher frequency for transmission of the wave.

Wireless Communication:

Wireless communication requires propagation of waves through the atmosphere without wires. The transmission of the wave at audio frequency becomes impractical because its efficiency of radiation is poor. Thus the transmission should be done at a higher frequency for obtaining higher radiation efficiency.

Types of Carrier Waves:

There is a need for translating the original low-frequency baseband message or information signal into the high-frequency wave before transmission such that the translated signal continues to possess the information contained in the original signal. In doing so, we take the help of a high-frequency signal, known as the carrier wave, and a process known as modulation which attaches information to it. The carrier wave may be continuous (sinusoidal) or in the form of pulses.

Continuous or Sinusoidal wave:

Pulse:

Types of Modulation w.r.t. Continuous (sinusoidal) Wave as Carrier Wave:

A sinusoidal carrier wave can be represented as

c(t ) = A_c sin (ωt +Φ)

where c(t) is the signal strength (voltage or current), A_c is the amplitude,

ω  = 2πf_c =  the angular frequency and

Φ is the initial phase of the carrier wave.

During the process of modulation, any of the three parameters, viz amplitude A_c, angular frequency ω, and the phase Φ, of the carrier wave can be varied with baseband signal or message or information signal. This results in three types of modulation.

• Amplitude modulation (AM): In this type of modulation, angular frequency ω and the phase Φ are kept constant and amplitude A_c of the carrier wave is varied in accordance with the modulating wave.
• Frequency modulation (FM): In this type of modulation, amplitude  A_c and the phase Φ are kept constant and angular frequency ω of the carrier wave is varied in accordance with the modulating
• Phase modulation (PM): In this type of modulation, amplitude A_c and angular frequency ω are kept constant and the phase Φ of the carrier wave is varied in accordance with the modulating

Note that the frequency modulation and phase modulation of a carrier wave is collectively called the angle modulation. Analog communication of analog signals is done using continuous wave.

Types of Modulation w.r.t. Pulse as Carrier Wave:

The significant characteristics of a pulse are pulse amplitude, pulse duration or pulse width, and pulse position (denoting the time of the rise or the fall of the pulse amplitude. Hence, different types of pulse modulation are:

• pulse amplitude modulation (PAM),
• pulse duration modulation (PDM) or pulse width modulation (PWM), and
• pulse position modulation (PPM)

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